Methacrylic acid and its esters are valuable raw material monomers for many polymerizations. The acids or its esters are normally prepared from acetone and hydrogen cyanide according to the so-called cyanohydrin method. Acetone cyanohydrin is reacted in concentrated sulfuric acid to obtain the methacrylamide sulfate. The continued reaction to methacrylic acid or its esters is likewise a generally practiced industrial procedure (Stanford Research Institute SRI, vol. 11, pages 27 et seq.). In this synthesis, ammonium sulfate is necessarily formed together with methacrylic acid or its esters. This is an obnoxious accompanying product, usable to a limited extent as a fertilizer. In addition to the formation of this by-product, this process is disadvantageous in that it requires working with poisonous hydrogen cyanide in the first stage. These facts have given rise to numerous efforts to provide a different method of synthesis.
Attempts have been made to oxidize isobutylene by way of methacrolein to obtain methacrylic acid. The yields obtained thus far, and the technical expenses involved in this two-stage gaseous phase oxidation process (SRI Report 11: 35-37) are not as yet adequate to provide a general, large-scale industrial process based on isobutene.
It has furthermore been proposed first to prepare tertbutanol from isobutene, then to react this alcohol in the gaseous phase to methacrolein, and to convert the latter to methacrylic acid (Hydrocarbon Processing, Feb. 1979, pages 105-107). Thus far, this process has likewise been unsuitable as a substitute for the cyanohydrin process.
Finally, several other processes have become known (Stanford Research Institute Report, 11: 30) for oxidizing isobutene with nitric acid or a mixture of nitric acid and nitrogen dioxide, or mixtures thereof with acetic acid, to obtain .alpha.-hydroxyisobutyric acid, a precursor of methacrylic acid. By splitting off water, methacrylic acid is obtained from .alpha.-hydroxyisobutyric acid (German Pat. No. 1,568,948=British Pat. No. 1,080,473 and Canadian Pat. No. 771,714; DOS No. 1,768,253=British Pat. No. 1,179,987).
Although these methods produce, in part, quite satisfactory yields based on butene, the reaction solutions and intermediate products are explosive. Furthermore, the nitrogen-oxygen compound involved is reduced to N.sub.2 or N.sub.2 O, rather than to NO. However, only NO, in contrast to N.sub.2 or N.sub.2 O, can be reoxidized with atmospheric oxygen and recycled as nitric acid, thus providing desirable economy.
In the past, efforts have also been expended to dehydrogenate isobutyric acid to methacrylic acid (German Pat. No. 2,129,920=British Pat. No. 1,332,558; German Pat. No. 2,208,580=British Pat. No. 1,360,550). This procedure likewise has failed to produce technically satisfactory results.
Therefore, great interest still exists in finding an improved process for preparing methacrylic acid from readily accessible starting materials in a technically simple way and in good yields.